1. Field of the Invention
The present invention relates to a radio apparatus and a gain control method, and more particularly to a radio apparatus carrying out Automatic Gain Control (abbreviated as AGC hereinafter) for a signal from a mobile terminal unit in a mobile communication system, and a gain control method for adjusting a gain convergence rate of AGC in the radio apparatus.
2. Description of the Background Art
In a mobile communication system (for example, Personal Handyphone System, abbreviated as PHS hereinafter) which has been rapidly developed in recent years, an approach to extract a signal from a desired mobile terminal unit through adaptive array processing in a radio receiving system at a base station is proposed in communication between the base station and the mobile terminal unit.
The adaptive array processing is processing to remove an interference component and correctly extract a signal from a desired mobile terminal unit by calculating a weight vector consisting of a reception coefficient (weight) for each antenna at a base station based on a received signal from a mobile terminal unit, for adaptive control.
The adaptive array processing requires a plurality of spatially distributed antennas, that is, an array antenna. In an array antenna, for example, formed of two antennas, an array output signal Y(t) is expressed as follows:
Y(t)=W1X1(t)+W2X2(t) 
where X1(t), X2(t) each represents a received signal at each antenna, and W1, W2 each represents a weight at each antenna.
The received signal at each antenna is expressed as follows:
X1(t)=H11S1(t)+H12S2(t)+n1(t) 
X2(t)=H21S1(t)+H22S2(T)+n2(t) 
where S1(t) represents a signal from a desired mobile terminal unit and S2(t) represents a signal from an interference-causing mobile terminal unit.
Here, H11 represents a propagation path property from the desired mobile terminal unit to antenna 1, and H12 represents a propagation path property from the interference-causing mobile terminal unit to antenna 1. H21 represents a propagation path property from the desired mobile terminal unit to antenna 2, and H22 represents a propagation path property from the interference-causing mobile terminal unit to antenna 2. In addition, n1(t) represents noise at a receiving system of antenna 1, and n2(t) represents noise at the system of antenna 2. The array output in this case is expressed as follows:                               Y          ⁡                      (            t            )                          =                  xe2x80x83                ⁢                                            (                              W1H11                +                W2H21                            )                        ⁢                          S1              ⁡                              (                t                )                                              +                                                  xe2x80x83                ⁢                                            (                              W1H21                +                W2H22                            )                        ⁢                          S2              ⁡                              (                t                )                                              +                                                  xe2x80x83                ⁢                              W1n1            ⁡                          (              t              )                                +                                    W2n2              ⁡                              (                t                )                                      .                              
Here, it is assumed that the weight that satisfies the following equation can be calculated:
(W1H11+W2H21)=1 
(W1H21+W2H22)=0. 
Accordingly, the array output signal can be expressed as follows:
Y(t)=S1(t)+n(t) 
where n(t)=W1n1(t)+W2n2(t).
Thus, the interference component can be removed and the signal can be received from the desired mobile terminal unit by calculating the appropriate weight through the adaptive array processing.
FIG. 11 is a functional block diagram functionally illustrating a radio apparatus that is provided for each antenna at a conventional base station carrying out the adaptive array processing using such a plurality of antennas and carries out AGC for a signal from a mobile terminal unit. FIG. 12 is a flow chart illustrating a gain control method for adjusting a gain convergence rate of AGC in such a radio apparatus.
First referring to FIG. 11, a signal received from an antenna 1 is amplified by an AGC amplifier 2, converted into an IQ signal formed of an In-phase component (I component) and Quadrature component (Q component) by a quadrature detector 3, and thereafter stored into memory 4.
IQ signal once stored in memory 4 is provided to a demodulation circuit 5. Demodulation circuit 5 also receives IQ signal from another antenna (not shown), performs the adaptive array processing as described above and demodulation processing and extracts the signal from each mobile terminal unit.
A reception level detection unit 6 obtains the reception level of the signal from IQ signal stored in memory 4. An amplitude value of IQ signal is calculated, for example, for eight symbols from 60th symbol of the received signal for each frame. The maximum amplitude value for these eight symbols is considered as the reception level for that frame.
A feedback data calculator 7 calculates feedback data that decides an amplitude ratio of AGC amplifier 2 in the next frame by the reception level obtained by reception level detection unit 6 and a step constant stored in memory 9. Here, when the reception level obtained by reception level detection unit 6 is represented by P_max, a prescribed ideal value is represented by P_ideal, and the step constant is represented by Step, the amount of change xcex94FB from feedback data at the time of receiving the previous frame to feedback data at the time of receiving the next frame can be calculated by the following equation:
xcex94FB=(P_maxxe2x88x92Pxe2x80x94ideal)/2Step. 
When the value of the feedback data at the time of receiving the previous frame is represented by FB, feedback data FBxe2x80x2 at the time of receiving the next frame can be expressed as follows:
FBxe2x80x2=FBxe2x88x92xcex94FB. 
The feedback data calculated by feedback data calculator 7 is once stored in memory 8. The stored feedback data is read in the next frame and provided to a gain control input of AGC amplifier 2 to be reflected in AGC at the time of receiving the next frame.
Referring to FIG. 12, the gain control method for adjusting the gain convergence rate of AGC in the radio apparatus shown in FIG. 11 will now be described. It is noted that the following process is implemented by a Digital Signal Processor (DSP) of the radio apparatus in a software manner.
At step S1, the signal from the mobile terminal unit is subjected to quadrature detection. Here, an RXIF signal that is an intermediate frequency signal received from the mobile terminal unit is converted to an RXIQ signal subjected to quadrature detection.
At step S2, such a symbol is set in that the reception level of the signal received from the mobile terminal unit starts to be detected. An example is herein shown where the reception level is detected from 60th symbol of the received signal.
At step S3, it is determined whether the present symbol is in a symbol period in which the reception level of the signal from the mobile terminal unit is detected. For example, when the reception level is detected in the 8-symbol period from the 60th symbol to the 67th symbol, if the symbol in which amplitude will be calculated from now on precedes the 68th symbol, a process of calculating the amplitude in that symbol will follow, and if not, a process of calculating the feedback data will follow.
At step S4, the amplitude in that symbol is calculated. The value of squared I component of IQ signal is added to the value of squared Q component of IQ signal. Here, in order to simplify the process, the square roots of the resulting sum is not obtained.
At step S5, it is determined whether or not amplitude A calculated at step S4 is greater than the maximum amplitude A_max stored until now.
At step S6, if it is determined that A is greater than A_max at step S5, A_max is replaced by A.
At step S7, the symbol in which the amplitude is calculated is shifted by one in ascending order.
At step S8, the amount of change of the feedback data is calculated from the maximum amplitude value of the amplitudes from the 60th symbol to 67th symbol and from a fixed step constant stored in the memory.
At step S9, the feedback data in the next frame is calculated and the calculated feedback data is reflected in AGC at the time of receiving the next frame.
In the adaptive array processing, weights are calculated such that the power of interference component is small when the signals received from a plurality of antennas are multiplied by respective weights to be synthesized. Specifically, the operation of multiplying the weight means that the amplitude and phase of the signal received from each antenna are adjusted appropriately. Therefore, if the amplitude and phase information of the received signal suffers an error due to waveform distortion and the like, the adaptive array processing cannot fully function.
On the other hand, as to the input/output characteristics of general amplifiers, output at a certain level causes saturation, thereby creating a non-linear region. Therefore, in a case where the feedback data is calculated based on the fixed step constant as described above, for example, if the reception level of the signal from the mobile terminal unit is largely fluctuated by the effects of a distance between the base station and the mobile terminal unit, any possible obstacle, fading, and the like, the amplifier (AGC amplifier 2) at the base station enters into this non-linear region, leading to distortion of the received waveform, thereby resulting in that the adaptive array processing cannot fully function.
It is therefore an object of the present invention to provide a radio apparatus and a gain control method allowing for an adaptive array processing without a non-linear operation of an amplifier even under an environment in which a reception level of a signal from a mobile terminal unit largely fluctuates due to the effects of a distance between a base station and the mobile terminal unit, any obstacle, fading, and the like.
In accordance with the present invention, a radio apparatus carrying out AGC for a signal from a mobile terminal unit includes: a gain control feedback circuit performing AGC on the signal depending on a level of the signal; a fading rate estimation unit estimating a fading rate of the mobile terminal unit; and a step constant determiner determining an optimum step constant which decides a gain of the gain control feedback circuit depending on the fading rate estimated by the fading rate estimation unit. The gain control feedback circuit includes a feedback data calculator calculating feedback data which defines a gain of the gain control feedback circuit based on the signal level and the optimum step constant as determined by the step constant determiner.
Preferably, the fading rate estimation unit calculates a reception response vector of the signal from the mobile terminal unit and estimates a fading rate from a correlation value of the reception response vectors between frames.
More preferably, the step constant determiner decides an optimum step constant at the fading rate estimated by the fading rate estimation unit based on an association table of preliminarily obtained fading rates and step constants.
More preferably, the feedback data calculator calculates feedback data in a next frame by obtaining a value of difference between a value of the signal level and an ideal value, dividing the value of difference by a prescribed constant a number of times corresponding to the step constant, followed by multiplication by a prescribed coefficient, thereby calculating an amount of change in feedback data, and adding the amount of change to feedback data at the time of receiving a previous frame.
In accordance with another aspect of the present invention, a radio apparatus carrying out AGC for a signal from a mobile terminal unit includes: a gain control feedback circuit performing AGC on the signal depending on a level of the signal; a received signal strength detection unit detecting a received signal strength of the signal from the mobile terminal unit, at an antenna end of the radio apparatus; a feedback data conversion unit carrying out conversion from the received signal strength detected by the received signal strength detection unit to optimum feedback data which decides a gain of the gain control feedback circuit based on an association table of preliminarily obtained received signal strengths and feedback data; and a feedback data switch unit selecting the optimum feedback data converted by the feedback data conversion unit as an initial value of feedback data of the gain control feedback circuit.
In accordance with a further aspect of the present invention, a radio apparatus carrying out AGC for a signal from a mobile terminal unit includes a gain control feedback circuit performing AGC on the signal depending on a level of the signal. The gain control feedback circuit includes a first gain control circuit supplying first feedback data allowing the level of the signal to converge to an ideal value slowly, and a second gain control circuit supplying second feedback data allowing the level of the signal to converge to an ideal value fast. The radio apparatus further includes a received signal strength detection unit detecting a received signal strength of the signal from the mobile terminal unit, at an antenna end of the radio apparatus, and a feedback data switch unit selecting the first feedback data or the second feedback data depending on a detection result from the received signal strength detection unit.
Preferably, the feedback data switch unit makes a switch from the first feedback data to the second feedback data when fluctuation of the received signal strength detected by the received signal strength detection unit decreases to a prescribed threshold or below or increases to a prescribed threshold or above.
In accordance with yet another aspect of the present invention, a radio apparatus carrying out AGC for a signal from a mobile terminal unit includes a gain control feedback circuit performing AGC on the signal depending on a level of the signal. The gain control feedback circuit includes a first gain control circuit supplying first feedback data allowing the level of the signal to converge to an ideal value slowly, and a second gain control circuit supplying second feedback data allowing the level of the signal to converge to an ideal value fast. The radio apparatus further includes a new terminal detector detecting whether a signal is received from a new mobile terminal unit, and a feedback data switch unit selecting the first feedback data or the second feedback data depending on a detection result from the new terminal detector.
Preferably, the new terminal detector includes a demodulation circuit demodulating a specific signal from a new mobile terminal unit and an error determiner determining whether an error exists in the signal demodulated by the demodulation circuit and if no error exists, determining that a signal is received from a new mobile terminal unit.
More preferably, the error determiner determines that a signal has been received from a new mobile terminal unit if a unique word error does not exist in the signal demodulated in the demodulation circuit.
More preferably, the error determiner determines that a signal has been received from a new mobile terminal unit if the number of bit errors in a preamble portion is equal to or smaller than a prescribed threshold in the signal demodulated in the demodulation circuit.
More preferably, the gain control feedback circuit includes a reception level detection unit detecting the level of the signal. The reception level detection unit calculates an amplitude value for each IQ signal during a prescribed period of a received signal and regards a maximum amplitude value among the amplitude values as a reception level of the received signal.
More preferably, the gain control feedback circuit includes a reception level detection unit detecting the level of the signal. The reception level detection unit calculates an amplitude value for each IF signal during a prescribed period of a received signal and regards a maximum amplitude value among the amplitude values as a reception level of the received signal.
In accordance with another aspect of the present invention, a gain control method in a radio apparatus carrying out AGC for a signal from a mobile terminal unit includes the steps of: performing AGC on the signal depending on a level of the signal; estimating a fading rate of the mobile terminal unit; and determining an optimum step constant which decides a gain of AGC, depending on the estimated fading rate. The step of carrying out AGC includes the step of calculating feedback data which defines a gain of AGC, based on the signal level and the optimum step constant as determined.
Preferably, the step of estimating a fading rate includes the step of calculating a reception response vector of the signal from the mobile terminal unit and estimating a fading rate from a correlation value of the reception response vectors between frames.
More preferably, the step of determining an optimum step constant includes the step of deciding an optimum step constant at the estimated fading rate based on an association table of preliminary obtained fading rates and step constants.
More preferably, the step of calculating feedback data includes the step of calculating feedback data in a next frame by obtaining a value of difference between a value of the signal level and an ideal value, dividing the value of difference by a prescribed constant a number of times corresponding to the step constant, followed by multiplication by a prescribed coefficient, thereby calculating an amount of change of feedback data, and adding the amount of change to feedback data at the time of receiving a previous frame.
In accordance with a further aspect of the present invention, a gain control method in a radio apparatus carrying out AGC for a signal from a mobile terminal unit includes the steps of: performing AGC on the signal depending on a level of the signal; detecting a received signal strength of the signal from the mobile terminal unit at an antenna end of the radio apparatus; carrying out conversion from the detected, received signal strength to optimum feedback data which decides a gain of AGC based on an association table of preliminarily obtained received signal strengths and feedback data; and selecting the converted optimum feedback data as an initial value of feedback data.
In accordance with yet another aspect of the present invention, a gain control method in a radio apparatus carrying out AGC for a signal from a mobile terminal unit includes the step of performing AGC on the signal depending on a level of the signal. The step of performing AGC includes a first step of supplying first feedback data allowing the level of the signal to converge to an ideal value slowly, and a second step of supplying second feedback data allowing the level of the signal to converge to an ideal value fast. The method further includes the steps of detecting a received signal strength of the signal from the mobile terminal unit, at an antenna end of the radio apparatus, and selecting the first feedback data or the second feedback data depending on a detection result.
Preferably, the step of selecting feedback data includes the step of making a switch from the first feedback data to the second feedback data when fluctuation of the detected, received signal strength decreases to a prescribed threshold or below or increases to a prescribed threshold or above.
In accordance with a still further aspect of the present invention, a gain control method in a radio apparatus carrying out AGC for a signal from a mobile terminal unit includes the step of performing AGC on the signal depending on a level of the signal. The step of performing AGC includes a first step of supplying first feedback data allowing the level of the signal to converge to an ideal value slowly, and a second step of supplying second feedback data allowing the level of the signal to converge to an ideal value fast. The method further includes the steps of detecting whether a signal is received from a new mobile terminal unit, and selecting the first feedback data or the second feedback data depending on a detection result.
Preferably, the step of detecting a new mobile terminal unit includes the steps of demodulating a specific signal from a new mobile terminal unit, and determining whether an error exists in the demodulated signal and if no error exists, determining that a signal is received from a new mobile terminal unit.
More preferably, the step of determining whether an error exists includes the step of determining that a signal has been received from a new mobile terminal unit when a unique word error does not exist in the demodulated signal.
More preferably, the step of determining whether an error exists includes the step of determining that a signal has been received from a new mobile terminal unit if the number of bit errors in a preamble portion is equal to or smaller than a prescribed threshold in the demodulated signal.
More preferably, the step of performing AGC includes the step of detecting a level of the signal. The step of detecting a level of the signal includes the step of calculating an amplitude value for each IQ signal during a prescribed period of a received signal and regarding a maximum amplitude value among the amplitude values as a reception level of the received signal.
More preferably, the step of performing AGC includes the step of detecting a level of the signal. The step of detecting a level of the signal includes the step of calculating an amplitude value for each IF signal during a prescribed period of a received signal and regarding a maximum amplitude value among the amplitude values as a reception level of the received signal.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.